A piece of wood to cover our back garden drain (Kirkley NR33)

We are looking for a piece of wood large enough to cover our back garden drain...as you can see from the photo we had covered it with two old bits of wood but they have rotted away over the years. Ideally we want one large piece of wood circa 18" square, large enough to cover the hole and have some room to spare to weigh it down with a few bricks. Many thanks.